The extended radiation source has been monitored for tracking, weld monitoring and autofocusing with single-element or array detectors. Though the array detector can monitor the variation of the extended radiation source easily, it cannot be used with a single-core fiber.
On the other hand, it is quite difficult to monitor the variation of the extended radiation source with single-element detector. In the tracking, a single-element detector can monitor the motion of the extended radiation source, but the local variation inside of the extended radiation source has never been monitored with single-element detector.
The chromatic aberration of optics has been used in autofocusing (U.S. Pat. No. 4,992,859) and tracking (U.S. Pat. No. 5,444,236). In these U.S. patents, array detectors have been used as means for capturing the images. Therefore, these prior arts have the problem that they cannot be used with the single-core fiber.
Further, the chromatic signals from weld pool have been used in the weld monitoring (U.S. Pat. No. 5,155,329), but this patent does not use the aberration of the optics itself.